Time Machine / Time Capsule Security

I have wifi setup through Time Capsule and all of my family and friends connect through it. I have my computer's backups done through Time Machine and I needed a text file from my main computer while I was using someone else's notebook - which was connected to my wifi. I was able to go through the network, go back into Time Machine and copy the text file. I don't mind my friends and family connecting to my wifi but I don't think it's a great idea to have them access all the files on my computer. How can you prevent this access from being allowed?